Ok, let me start this out. If anyone can add to this (or correct my errors) please do so.
A Picatinny Rail was originally a scope mount that was attached to a barrel of a rifle as an add-on. It is a military standard size established by the Picatinny Arsenal just before the First World War (something like PR-MIL-1913, I think). It is a rail onto which a guide attached to a scope (and later a light or laser) can be attached by sliding the guide onto the rail and locking it in place with set screws.
The rail has notches or indentations in it so that it will not be affected as the heat from the barrel transfers to the metal of the rail. Now the picatinny is frequently seen slung inder a barrel for lasers or lights as well as the top mounting.
The Weaver rail was originally a proprietary rail from the Weaver Scope company. It typically attaches to the barrel or reciever via screws and taps into the metal. The scope has a guide attached that slides onto the rail and locks tight with a clamp-like action with screws or knobs.
When lasers and tac lights became common, many pistol manufacturers cut slots (rails) into the front of the frame to handle tac accessories. The most common size is that used on Glocks and other similar pistols like the CZ P01 and Springfield Armory XD firearms. Some have rails that are just cuts and some have indentations to allow the accessory to lock in place.
Walther has their own proprietary rail slots in the pre-'04 pistols--tac accessories from Glocks do not fit it. I understand that new Walthers used a tac rail system similar to the Glock-type. There are other pistols that have proprietary systems; maybe someone else could point out which pistols use which mounts.
In short, standard sizes exist on Picatinny, Weaver, Glock and some other rail systems. Others like Pre-'04 Walthers are proprietary.
Most light systems have Weaver or Picatinny mounts that can be put on a firearm. There are also mounting systems that adapt proprietary rails to use the standard Glock system.
That is just about the extent of my knowledge on the subject.
I think the only clarification is that the 2004 P99's and beyond use the Picatinny standard -- not a "Glock" system. That doesn't mean all Picatinny-compatible accessories will work -- the distance between the trigger guard and the end of the rail limits your options (for example, the M3 SureFire light won't fit -- you need to use the M5).